Added some release notes

Summary: Some of these notes are important to our users, so adding them now to ensure they aren't missed during release.

Test Plan: Read it.

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4024

doc/release-notes.md

@@ -3,3 +3,10 @@ Bitcoin ABC version 0.20.2 is now available from:
   <https://download.bitcoinabc.org/0.20.2/>
 
 This release includes the following features and fixes:
+ - Improved initial block download performance by ~20%.
+ - Improved initial block download performance during pruning.
+ - `bitcoin-qt -resetguisettings` now generates a backup of the former GUI settings.
+ - Removed features that were deprecated in 0.19.x, including `signrawtransaction`,
+   `fundrawtransaction -reserveChangeKey`, parts of `validateaddress`, use of
+   addresses in `createmultisig`, and other miscellaneous behaviors.
+ - Minor logging improvements.